Interpretive Summary: Many of the commercially most significant crops in temperature North America (e.g., corn, soybean, cotton and others) are referred to as 'chilling sensitive.' Photosynthetic metabolism is among the most chill sensitive process in these plants and the chilling sensitivity of photosynthesis plays a critical role both in limiting the geographical range where these crops are grown as well as accounting for the annual variation in the economic success of these crops grown at the northern border of their cultivation. An improvement of even one degree in the low temperature tolerance would have a far reaching beneficial impact on the agronomy of these important crop species. Our earlier work demonstrated that chilling interferes with the regulation of sucrose phosphate synthase, a key regulatory enzyme of leaf carbohydrate metabolism and interrupts the internal timing mechanism (i.e., circadian rhythm) of chilling sensitive plants. Our current work reveals that the different mechanism regulation of sucrose phosphate synthesis in highly cold tolerant antarctica grass is a key aspect of this species cold tolerance. This finding is an important clue in understanding the molecular basis for the chilling sensitive of photosynthesis in crop plants and of interest to agricultural researchers working to improve chilling tolerance.

Technical Abstract:
Deschampsia antarctica, a freezing-tolerant grass that has colonized the Maritime Antarctic, has an unusually high content of sucrose in leaves, reaching up to 36% of dry weight. Sucrose (Suc) accumulation has often been linked with increased sucrose phosphate synthase SPS (EC: 2.4.1.1.14) activity. SPS, a key enzyme in sucrose biosynthesis, is controlled by an intricate hierarchy of regulatory mechanism including allosteric modulators, reversible covalent modification in response to illumination, and
transcriptional regulation. We hypothesized that during long day conditions in the Antarctic summer D. antarctica can maintain high SPS activity longer by indirect light regulation, thereby leading to a high sucrose accumulation in the leaves. The objectives of this study were to investigate a possible indirect light regulation of SPS activity and the effect of cold and day length on transcriptional and protein level of SPS in D. antarctica. Although SPS activity did not display an endogenous rhythm of activity in continuous light, activation of SPS at the end of the dark period was observed in D. antarctica. This activation of SPS is possibly controlled by covalent modification, because it was inhibited okadaic acid while the SPS protein level did not significantly change. The highest SPS activity increase was observed after 21 days of cold-acclimation under LD treatment. This increased activity was not related to an increase of SPS gene expression or protein content. High SPS activity in cold long days appears to be among features that permit D. antarctica to survive in the harsh Antarctic conditions.